MyNokiaBlog has posted this video comparing the performance of the Nokia Lumia 620 vs the Nokia Lumia 920.
Strangely the Nokia Lumia 620 performs better in many ways, especially in boot-up times and app launch times.
It may be that the 620 has faster Flash storage, or that the lower RAM device has optimizations which is not activated on the Nokia Lumia 920.
It is well know that the Nokia Lumia 620 also has a more modern Adreno 305 GPU which is not present on the Nokia Lumia 920 (except for the 920T version in China) which has the much older Adreno 225 all of which suggests when Nokia releases their new mid and low-range Windows Phones at MWC 2013 performance issues will not be significant problems.
Thanks Pankaj for the tip.